Narrow stile metal doors of the type commonly used in retail stores and other commercial establishments typically have a minimum amount of metal frame around a central glass portion. This is done to provide an inviting and open look to the entry of the establishment, whether a single door is used or a set of double doors is employed. Such doors are highly susceptible to forceable unauthorized entry. Burglars prefer not to break the glass and rarely enter through the glass, because of the significant danger of severe injury which is possible when the glass shatters. In addition, many doors of this type use tempered glass which is difficult to break. The doors, however, are inherently weak at the point where the bolt enters the strike hole in the doorjamb. Burglars frequently employ crowbars or pry bars to rip out the strike hole or bend the door and doorjamb at the strike hole to open the door, even when the bolt is in its extended position. In many cases, this can be accomplished with relatively little effort and in an extremely short period of time.
Such commercial doors also frequently are made of aluminum, which is a relatively soft or easily bent metal. The lock cylinders can be pried or twisted out of the door, using conventional tools available in any hardware store or home workshop. In addition, the door may be drilled to trip the deadlocking mechanism for the lock. In some cases where the gap between the door and the doorjamb or between a pair of doors (where double doors are used) is relatively wide, and this occurs frequently, the door and doorjamb can be spread sufficiently in the area of the bolt to permit the door to be opened, again without removing or retracting the extended bolt. Unfortunately, burglaries of commercial establishments are constantly increasing; so that it is important to provide a lock for such doors while, at the same time, retaining the open appearance which is provided by the large glass inserts normally used.
A relatively simple door protector device for inhibiting the spreading apart of the door and doorjamb and for providing protection over the area where the lock bolt enters the doorjamb is disclosed in the patent to Bennett U.S. Pat. No. 3,761,119. This comprises a protective plate which is fastened through carriage bolts onto the door and which extends over the space between the door and doorjamb. A pair of studs or pins also extend into holes in the doorjamb when the door is closed. The studs are secured to the face of the plate to project inwardly into the holes in the doorjamb. Thus, when the door is closed and locked in place, access to the bolt is prevented by the plate which overlies the bolt. The pins and carriage bolts which hold the plate onto the doorjamb and the door, respectively, prevent the doorjamb from being forced apart from the door in the area of the bolt. While this device provides a relatively good degree of protection, the knob or lock on the door is exposed; and a burglar with a knowledge of the construction of the device could pry the portion overlying the doorjamb away from the doorjamb, thereby defeating the purpose of the studs, unless the plate is made of extremely rigid material to prevent bending. Another door lock guard which is similar to the one of Bennett patent is disclosed in the patent to Hennessy U.S. Pat. No. 4,484,463. This device also comprises a plate which covers the space where the bolt extends between the door and doorjamb to prevent access to the bolt through this space. The plate has a thick center portion with a hole in it adapted to fit over the lock cylinder, so that access to the lock cylinder is prevented. This minimizes the potential for removal of the lock cylinder from the door. Carriage bolts or the like are used to fasten the plate to the door, but there is no provision in this patent for any physical holding of the plate onto the doorjamb.
One of the earliest examples of a plate which extends over the area of the bolt and which engages the doorjamb is disclosed in the patent to Miller U.S. Pat. No. 695,472. An escutcheon plate extends around the key opening and over the space between the door and the doorjamb to overlap the doorjamb. A projection is provided on the back surface of the escutcheon plate to engage a mating hole in the doorjamb when the door is closed and locked. This prevents the prying apart of the door from the doorjamb; and the plate itself covers the area where the bolt is; so that access to the bolt also is prevented by the plate.
Other approaches to making locks burglar resistant are directed to protective plates which extend over the lock cylinder or the area in which the lock cylinder is mounted on the door. Typically these plates are made of treated metal which is resistant to drilling and have thicknesses selected to prevent potential burlgars from having any direct contact with the edges of the lock cylinder to make it more difficult to remove the lock cylinder from the door.
For those prior art devices which protect the lock cylinder, but leave the opening between the door and the doorjamb unprotected, practically no protection from a burglar with a pry bar exists for most commercial business doors, since the door itself can be pried away from the doorjamb in the manner described previously. For those devices which include pins extending into the doorjamb, it is necessary to make the guard plate of expensive, high strength, treated stainless steel or the like for the plate to be effective. If this is not done, it is possible to pry the guard plate away from the doorjamb and then proceed as if the plate was not in place to separate the doorjamb from the door or to remove the strike hole from the doorjamb. Such plates are relatively difficult to manufacture and machine and are relatively expensive.
It is desirable to provide a door lock guard device which overcomes all of the shortcomings of the prior art and which significantly increases the degree of protection provided, by positively interlocking the door and doorjamb through the guard plate, irrespective of the condition of operation of the primary locking bolt.